Superposition of ballistic on steady contractions in man

In a visual reaction time task, human subjects superimposed isometric ballistic contractions on a maintained activity in the soleus or anterior tibial muscle. Since there were good reasons to believe that the supraspinal motor commands for the ballistic contractions were independent of those for the background activity, the interaction between the motor commands for the ballistic and for the steady contractions could be studied at the spinal level. If ballistic and steady contractions were in the same direction, the EMG burst and torque changes associated with the ballistic contraction were nearly constant irrespective of the maintained steady flexion force. This was true if a muscle was activated to about 5% of its maximum force as the soleus muscle during plantar flexions and if it was activated to about 40% of its maximum force as the anterior tibial muscle during dorsal flexions. If ballistic and steady contractions were in opposite directions the torque changes related to the ballistic contraction increased linearly with the background activity. This relation was caused by a reduction in antagonist activity starting about 50 ms before the agonist EMG burst and not by an increased agonist burst, the latter remaining independent of background activity. These results imply that the input-output relationship of the motoneuronal pool is nearly linear. The functional basis of this relation is the size principle which is valid during continuous and ballistic contractions. The number of motor units recruited for the ballistic contraction is adjusted according to their force such that the contraction amplitude remains constant.